Communications in space requires the acquisition and tracking of an electromagnetic signal source by a signal receiver. Such systems can also be used to track stars, beacons, satellites, etc. While such systems typically use light as the electromagnetic signal to be tracked, it will be understood that, signals outside the frequency bands of visible light could be tracked, such as X-ray signal sources, etc., assuming an appropriate signal detector is used for the frequencies of interest. While optical signals will be described herein, this should be construed to include other signals having the characteristics of a beam that can be completely imaged within the bounds of an appropriate detector array.
An optical acquisition and tracking system is described in the paper "Pointing, Acquisition and Tracking For Optical Space Communications", by G. Baister and P. V. Gatenby, Electronics & Communication Engineering Journal, December 1994, pages 271-280. This system uses gimbals to point an optical detector array in coarse and fine pointing loops, and an electronic quadrant detector to define the position of the signal on the array and provide signals which are used to control the gimbals.
The quadrant detector achieves high precision only within a restricted field of view, requiring precise adjustment of the system optics, and precise alignment of the acquisition and tracking system to the user terminal. Long-term stability of all adjustments and alignments are critical, and offset tracking from the optical boresight by an arbitrary angle is not possible.
Detection of an optical signal by a charge coupled device (CCD) array, and processing though a Fourier transform of the resulting signal has been described in U.S. Pat. No. 5,343,287 issued Aug. 30, 1994 to Wilkins, for determining atmospheric turbulence and angle of arrival changes in a laser communications signal. This system obtains, from the Fourier processed signal, an airy disk (a bright spot at the center of diffraction rings), to determine the signal location. The system is directed to a communication link having at least one end within the atmosphere, which diffuses and distorts the signals. The signals which it processes are typically strong and reliable. It cannot determine the location of a signal source to the precision typically required in systems permanently operated above the atmosphere, which signals can be extremely weak, and cannot tolerate variations in those signals with the tolerances typically obtained in atmosphere-corrected signals. Due to the nature of the system, it is difficult to track and acquire weak signals.